As a method of observing magnetic domains of a ferro-magnetic substance with an electron microscope, a differential phase contrast (DPC) scanning transmission electron microscopy using a scanning transmission electron microscope (STEM) is described in Optik, West Germany, 67, No. 4 (1984), pp. 363-376.
The differential phase contrast scanning transmission electron microscopy is a straightforward method for creating an image from information about phase contrast. As an example, a detector that is divided into two portions is used to detect a transmitted electron beam. The difference between the output signals from the two detector portions is supplied as an image signal to a cathode-ray tube. As a result, a scanning transmission electron micrograph is obtained. This method can give rise to excellent images when phase contrast is created from topographical features or magnetic domains which images are difficult to create by any other method.
FIG. 1 schematically shows the electron optical system of a scanning transmission electron microscope. An electron beam emitted by an electron gun G is focused onto a specimen 3 by a condenser lens 2a, objective lens 2b disposed ahead of the specimen 3. A scan coil 1 which receives a scanning signal from a scanning circuit 22 causes the focused beam to scan an area of the surface of the specimen 3. When the electrons of the beam pass through the specimen 3, they are scattered in various directions. As a result, a diffraction image of the specimen is created by an imaging objective lens 4 in the back focal plane 5 of the objective lens 4. The position of the focused diffraction image is independent of the irradiated position on the specimen 3 but depends on the directions in which the electrons are scattered, as shown in FIG. 2. In FIG. 2, electrons scattered through angle .theta. by the specimen 3 are focused at position A, irrespective of where they impinged upon the specimen 3. Electrons scattered through angle .phi. by the specimen 3 are focused at position B. Those electrons which were not scattered by the specimen 3 and traveled straight are focused at position 0. Thus, it can be seen that the diffraction image focused onto the back focal plane 5 contains information about the phase difference created by the specimen 3.
Then, the diffraction image focused onto the back focal plane is magnified and projected onto two detectors 8 and 9 by an intermediate lens 6 and a projector lens 7 which are disposed behind the back focal plane. At this time, an electron micrograph of the specimen 3 is imaged at a reduced magnification between the projector lens 7 and the array of the detectors 8, 9.
Referring again to FIG. 1, the specimen shown has magnetic domains. It is now assumed that the electron beam falls on neighboring magnetic domains 10 and 11 whose magnetization directions have an antiparallel relation to each other. The paths of electron rays passing through the magnetic domains 10 and 11 are indicated by the solid lines and the broken lines, respectively. The bold, solid and light, solid lines indicate rays deflected by the Lorentz force produced by the magnetic domains, while the solid and the broken line indicates undeflected rays.
When the electron beam hits the right magnetic domain 10, the diffraction image is shifted toward the detector 9 as indicated by 20 in FIG. 3(a). When the beam impinges on the left domain 11, the diffraction image is shifted toward the detector 8 as indicated by 21 in FIG. 3(b). Therefore, if a signal proportional to the difference between the output signal A from the detector 8 and the output signal B from the detector 9 is produced by a differential amplifier 23, the signal components contributed to the phase difference are summed up, whereas the signal components which are not related to the phase difference are canceled out by the amplifier 23, because the magnitudes of the latter components detected by both detectors remain the same (see the solid and the broken lines in FIG. 1).
Thus, the scan coil 1 causes the electron beam to scan the surface of the specimen in two dimensions. The differential amplifier 23 produces a signal proportional to the difference between the output signal A from the detector 8 and the output signal B from the detector 9. The differential signal from the amplifier 23 is fed as an image signal to a cathode-ray tube 24 synchronized with the scan of the electron beam on the specimen. In this way, a differential phase contrast image Z is presented on the viewing screen of the CRT 24. This image Z contains various levels of brightness, corresponding to various magnetization directions of the magnetic domains produced on the specimen.
The detector can be split into two as shown in FIGS. 3(a) and 3(b) or into four as shown in FIG. 3(c). In the latter case, output signals A, B, C, D from detectors 15, 16, 17, 18, respectively, are so treated that the calculation given by (A+D)-(B+C), for example, is performed to obtain a phase difference.
In order to obtain a clear differential phase contrast image, it is desired that diffraction images 20 and 21 be symmetrically projected onto the two detectors 8 and 9, as shown in FIG. 4(a). Although a differential phase contrast image can be observed even if the diffraction images 20 and 21 deviate slightly from the centers of the detectors 8 and 9, respectively, as shown in FIG. 4(b), the asymmetry leaves behind unwanted information after the arithmetic operation and, therefore, it is impossible to derive a clear differential phase contrast image. Especially, where the diffraction images 20 and 21 are projected only onto the detector 8 as shown in FIG. 4(c), it is impossible to obtain any differential phase contrast image.
One conceivable method of solving the foregoing problems consists in mounting split detectors movably and rotatably and adjusting their positions and orientations relative to the diffraction images so that the separated diffraction images may be symmetrically projected on the split detectors. However, it is impossible to confirm how the diffraction images are projected onto the detectors and so no one knows how to adjust the positions and the orientations of the detectors. Consequently, it is difficult to put this method into practical use.